Optical Transport Network (OTN) refers to a transport network to perform optical area processing of signals at the client layer. The OTN functions include transmission, multiplexing, routing, monitoring, and survivability functions. A main feature of the OTN is that the concept of optical layer is introduced. The optical layer is added between electrical multiplexing layer and physical layer of a Synchronous Digital Hierarchy (SDH) transport network. The most basic object processed by the OTN is optical wavelength. The services at the client layer are multiplexed, transmitted, routed, and amplified in the form of optical wavelength on an optical network, and add-drop multiplexing and cross connection of the services are performed in an optical domain to provide effective and reliable transmission of the signals of a customer. In the hierarchical structure of the OTN, the OTN is broken down into several independent networks in layers. The optical layer is classified into the Optical CHannel (OCH) layer, Optical Multiplexer Section (OMS) layer, and Optical Transport Section (OTS) layer. Each hierarchical network can be further segmented into sub-networks and links between sub-networks to reflect the internal structure of the network at the layer.
The OCH is responsible for selecting routes, allocating wavelength, and arranging connection for signals in different formats at the client layer, thus providing the end-to-end optical channel interworking function. The function includes: re-arranging optical channel connection to flexibly implement routing of the network; processing the optical channel overhead to ensure the integrity of optical channel adaptation information; and providing the optical channel monitoring function for the running and management of the network layer. The OCH is further classified into the Optical channel Payload Unit (OPU), Optical channel Data Unit (ODU), and OTU. The subdividing of OCH into sub hierarchies is to meet the requirements for multi-protocol services that are adaptive to optical network transmission and for network management and maintenance.
FIG. 1 shows a structure of an OTN frame with the existing technology. The OTN frame format with the existing technology is defined as follows:
The OTN frame includes the Optical channel Payload Unit-k (OPUk) payload, the Forward Error Correction (FEC) part of the Optical channel Transport Unit-k (OTUk), and the overheads produced by transmitting the service data payload. The overheads include the Optical channel Payload Unit-k Overhead (OPUk OH) in columns 15 and 16, Optical channel Data Unit-k Overhead (ODUk OH) in columns 1 to 14 in rows 2 to 4, and Optical channel Transport Unit-k Overhead (OTUk OH) in columns 8 to 14 in row 1. The OTUk is called a thoroughly standardized OTU. k represents a rate level, where, k=1 represents 2.5 Gbit/s, k=2 represents 10 Gbit/s, and k=3 represents 40 Gbit/s.
When the OTN is widely used, services at various rates need to be mapped into a high-rate OTUk network for transmission; and the high-rate OTUk needs to de-map the service data to acquire multiple routes of low-rate services.
With the existing technology, the OTN multiplexing and mapping are implemented in the following mode: a Positive Justification Opportunity (PJO) and a Negative Justification Opportunity (NJO) are set in a high-rate OTUk to compensate the difference between high-rate OTUk and low-rate OTUj clock, thus implementing mutual conversion of OTN signals at various rate levels. As shown in FIG. 1, the OPUk OH is located in columns 15 and 16 of the OTUk frame. The part includes a Payload Structure Identifier (PSI) byte, a Justification Control (JC) byte, a Negative Justification Opportunity (NJO) byte, a Positive Justification Opportunity 1 (PJO1) byte, and a Positive Justification Opportunity 2 (PJO2) byte. The JC byte is used for explaining whether the data of NJO, JO1, and JO2 is effective. These justification bytes are used for adjusting transmitted clock information.
With the PJO and NJO methods, only services at some rate levels can be multiplexed and mapped into the OTN. Therefore, only several types of services can be mapped.